Ceratioidei

Seadevils, Devilfishes, Deep-sea Anglerfishes

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Introduction

Of the five major taxa of lophiiform fishes, the deep-sea Ceratioidei is the most phylogenetically derived (Bertelsen, 1984; Pietsch, 1984). Ceratioidei are distributed throughout the world’s oceans below a depth of 300 m. With 160 species, it constitutes by far the most species-rich vertebrate taxon within the bathypelagic zone and below—more than twice as many families and genera and more than three times the number of species as the Cetomimoidei, the next most species-rich deep-sea vertebrate taxon (see Paxton, 1998; Herring, 2002). At the same time, new species are being added to the suborder at a steady if not increasing rate.

Members of the group differ remarkably from their less-derived, bottom-living relatives by having an extreme sexual dimorphism (shared by all contained taxa) and a unique mode of reproduction in which the males are dwarfed—those of some linophrynids, adults at 6-10 mm standard length, competing for the title of world’s smallest vertebrates (see Winterbottom and Emery, 1981; Roberts, 1986; Weitzman and Vari, 1988; Kottelat and Vidthayanon, 1993; Watson and Walker, 2004)—and attach themselves (either temporarily or permanently) to the bodies of relatively gigantic females. In Ceratias holboelli, where the most extreme examples are found, females may be more than 60 times the length and about a half a million times as heavy as the males (Bertelsen, 1951; Pietsch, 1976, 1986). The males lack a luring apparatus and those of most species are equipped with large well-developed eyes (Munk, 1964, 1966) and relatively huge nostrils (Marshall, 1967a, b), the latter apparently used for homing in on a female-emitted, species-specific pheromone (Bertelsen, 1951; Pietsch, 1976; Munk, 1992). Normal jaw teeth are lost during metamorphosis, but are replaced by a set of pincer-like denticles at the anterior tips of the jaws for grasping and holding fast to a prospective mate. In some taxa male attachment becomes permanent and parasitic through fusion of male and female tissue (see Sexual Parasitism in Ceratioid Anglerfishes).

Ceratioid anglerfishes differ further from their shallow-water relatives in having a bacterial light-organ that serves as bait to attract prey, a structure technically called the “esca”—exceptions among members of the suborder include the monotypic family Neoceratiidae (Bertelsen, 1951), the three species of the gigantactinid genus Rhynchactis (Bertelsen et al., 1981; Bertelsen and Pietsch, 1998), and the five members of the family Caulophrynidae (Pietsch, 1979). Parr (1927) was the first to recognize the diagnostic value of the external morphology of escae in ceratioids, pointing out the need for a closer examination of individual variation in the structure of this organ. Since that time, differences in the number, shape, and size of escal appendages and filaments, as well as variation in external escal pigment patterns, have been, for the most part, the sole basis on which new species have been described (e.g., see Pietsch, 1974; Bertelsen et al., 1981; Bertelsen and Krefft, 1988).

The internal structure of ceratioid escae is infinitely more complex, involving a confusing array of bacteria-filled vesicles, light-absorbing pigment layers, reflecting tissues, tubular light-guiding structures, nerves, blood vessels, and smooth muscle fibers (Munk and Bertelsen, 1980; Munk, 1988, 1998, 1999; Herring and Munk, 1994; Munk and Herring, 1996; Munk et al., 1998). There is some evidence also that ceratioid escae contain pheromone-producing secretory glands that function to attract a conspecific male (Munk, 1992), but the true nature and adaptive significance of these structures and most of the other internal parts of escae are unknown.

In addition to the esca, all 23 currently recognized species of the ceratioid genus Linophryne (family Linophrynidae) bear an elaborate bioluminescent hyoid barbel, the light of which does not originate from symbiotic luminescent bacteria but rather from a complex array of intrinsic, intracellular, paracrystalline photogenic granules; the bacteria-filled esca is ectodermal in origin, whereas the barbel light organ appears to be derived from the mesoderm (Hansen and Herring, 1977). This remarkable dual system, involving two entirely separate mechanisms of light production, is unique among animals.

In summary, ceratioid anglerfishes are among the most intriguing of all animals, possessing a host of spectacular morphological, behavioral, and physiological innovations found nowhere else. The suborder is taxonomically diverse: with 160 currently recognized species (and many more certain to be discovered in the future), it forms a major contribution to the biodiversity of the deep-sea. It is exceedingly widespread geographically, occurring in deep waters of all major oceans and seas of the world, from high Arctic latitudes to the Southern Ocean; while some species appear to be almost cosmopolitan in distribution, many others have surprisingly small, restricted, vertical and horizontal ranges. Their relative abundance, high species diversity, and trophic position as the top primary carnivores in meso- and bathypelagic communities make them important ecologically. Their unique mode of reproduction has significant biomedical implications to the fields of endocrinology and immunology. Yet, despite these many aspects of biological interest and importance, as well as a large amount of revisionary work published in the 1970s and early 1980s, including repeated attempts to resolve phylogenetic relationships, ceratioid anglerfishes have remained poorly known.

Characteristics

Diagnosis

A suborder of lophiiforms unique and derived in lacking pelvic fins (except in larval and newly metamorphosed Caulophrynidae), pelvic bones reduced or absent; extremely sexually dimorphic, males dwarfed, a fraction of size of females. Spinous dorsal-fin of two spines (the illicium and second cephalic spine), both supported by a single pterygiophore; anteriormost dorsal-fin spine modified to serve as a lure, emerging from dorsal surface of cranium (somewhat behind cranium in Bufoceratias, from roof of mouth in Thaumatichthyidae, absent in Neoceratiidae); second dorsal-fin spine reduced to a tiny remnant (but relatively well developed in adolescent Diceratiidae and Ceratiidae, absent in Neoceratiidae and Linophrynidae), embedded beneath skin of head and lying on, or fused to, dorsal surface of pterygiophore just behind base of illicial bone; postcephalic, spinous dorsal-fin absent; palatine teeth absent; interhyal without medial, posterolaterally directed process; ceratobranchial V toothless, reduced to a slender rod-shaped element in most families, represented by tiny remnants in the Gigantactinidae; epibranchial I simple, without ligamentous attachment to epibranchial II; pharyngobranchial IV absent; cleithral spine absent; interopercle reduced, elongate and narrow; subopercle with ascending process absent or reduced to a small spine or projection detached from opercle; gill filaments present as holobranchs on gill arches II and III, extending as hemibranchs onto proximal end of arch I in some families (i.e., Caulophrynidae, Himantolophidae, Melanocetidae, Diceratiidae, some Oneirodidae, Centrophrynidae, and Neoceratiidae), and as hemibranchs on arch IV; pseudobranch absent (tiny remnants present in some Gigantactis); swimbladder absent.

Males with elements of cephalic dorsal-fin spines greatly reduced, remnants embedded beneath skin of head; eyes and olfactory organs greatly enlarged in most taxa; tip of snout and chin bearing hooked denticular teeth, used for attachment to females; in some families and genera attachment becomes parasitic through fusion of male and female tissue.

Description

Females with body short and deep, almost spherical in most families, but elongate and somewhat laterally compressed in Thaumatichthyidae, Centrophrynidae, Ceratiidae, Gigantactinidae, Neoceratiidae, and some Oneirodidae; head length usually greater than 40% SL, but only about 25% in elongate species; mouth moderate to extremely large (compared to other lophiiforms), length of premaxilla as little as 10% SL (in some Gigantactinidae) to as great as 40% SL (in some Linophrynidae); cleft of mouth nearly horizontal (e.g., Thaumatichthyidae, Centrophrynidae, and Gigantactinidae) or oblique to almost vertical (e.g., Melanocetidae and Ceratiidae).

Sexual dimorphism extreme: males dwarfed, those of most genera reaching a maximum standard length of only 6-8% of that of females, and less than 25% of that of the largest females of genera for which mature females are known; body elongate, head less than 40% SL; mouth small, length of premaxilla usually less than 15% SL; cleft of mouth horizontal. Metamorphosed males of all families with hooked denticular teeth apparently adapted especially for gripping the skin of females; becoming permanently attached and parasitic in Caulophrynidae, Ceratiidae, Neoceratiidae, Linophrynidae, and the oneirodid genera Bertella and Leptacanthichthys.

Fin-ray counts the same for males and females, highly variable: Dorsal rays 3-8 in most families, 11-13 in Neoceratiidae, and 12-22 in Caulophrynidae and Melanocetidae; anal rays 3-7 in most families, 5-19 in Caulophrynidae, and 10-13 in Neoceratiidae; caudal rays nearly always 9, but 8 in the ceratiid genus Cryptopsaras and 10 in some specimens of Neoceratiidae; pectoral rays 12-30. Pelvic fins absent (except in larval and early metamorphosis stages of Caulophrynidae). Rays of unpaired fins sometimes greatly elongate (e.g., in females of Caulophrynidae and Gigantactinidae) and often not interconnected, or interconnected with extremely thin, transparent, membrane-like tissue. Caudal fin rounded in nearly all families, but emarginate in some Gigantactinidae; caudal rays of some large individuals of Ceratiidae terminating in spherical skin-covered ossifications.

Coloration of males and females usually uniform dark brown to black over entire head, body, and fins; dermis everywhere unpigmented and translucent in the linophrynid genus Haplophryne.

Sexual Parasitism in Ceratioid Anglerfishes

In some ceratioid taxa, the male's attachment to the female is followed by fusion of epidermal and dermal tissues and, eventually, by a connection of the circulatory systems so that the male becomes permanently dependent on the female for blood-transported nutrients, while the host female becomes a kind of self-fertilizing hermaphrodite (Regan, 1925a, b, 1926; Parr, 1930; Regan and Trewavas, 1932; Bertelsen, 1951; Pietsch, 1975, 1976; Munk and Bertelsen, 1983; Munk, 2000). Permanent attachment is usually accomplished by means of separate outgrowths from the snout and tip of the lower jaw of the male, both of which eventually fuse with the skin of the female. In some species a papilla of female tissue protrudes into the mouth of the male, sometimes appearing to completely occlude the pharynx. The heads of some males become broadly fused to the skin of the female, extending from the tip of the lower jaw to the rear of the skull, appearing as if embedded or absorbed by their mate, while in others, the male is carried at the tip of an elongate, cylindrical stalk of female tissue.

Increasing considerably in size once fused, their volume becoming much greater than free-living males of the same species, and being otherwise completely unable to acquire nutrients on their own, the males are considered to be parasites. They apparently remain alive and reproductively functional as long as the female lives, participating in repeated spawning events. A single male per female appears to be the rule in some taxa, but in others multiple attachments are relatively common, with as many as eight coupled to a single host (Saruwatari et al., 2001).

Since its discovery some 80 years ago (Saemundsson, 1922; Regan, 1925a, b), the story of sexual parasitism in ceratioid anglerfishes has become a part of common scientific knowledge. However, the known facts concerning this remarkable reproductive mode have never been thoroughly and satisfactorily analyzed, despite the work of Bertelsen (1951) and more recently of Munk and Bertelsen (1983), Munk (2000), and Pietsch (2005). The physiological mechanisms (endocrinological and immunological) that allow for sexual parasitism, which could be of significant biomedical importance, have never been explored.

Bertelsen, E., and T. W. Pietsch. 1977. Results of the research cruises of the FRV “Walther Herwig” to South America. XLVII. Ceratioid anglerfishes of the family Oneirodidae collected by the FRV “Walther Herwig.” Arch. FischWiss., 27(3): 171189.

Bertelsen, E., and T. W. Pietsch. 1996. Revision of the ceratioid anglerfish genus Lasiognathus (Lophiiformes: Thaumatichthyidae), with the description of a new species. Copeia, 1996(2): 401409.

Page: Tree of Life
Ceratioidei. Seadevils, Devilfishes, Deep-sea Anglerfishes.
Authored by
Theodore W. Pietsch and Christopher P. Kenaley.
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